I just encountered a problem with the adding MODEL and HIST units to the game, which seems reproducible, and which some modders may encounter if they add additional units ...

I've been adding units to flesh out (for a start) the Italian 8th Army and subordinate Corps with historical Army and Corps level assets and, at one point, though I was able to create a UNIT and then create a MODEL attached to it, and a HIST unit attached to the MODEL, when I placed a copy of the UNIT on the map and tried to connect it to the HIST unit ... the HIST unit would not appear on the drop down menu. The last unit on the list was the unit I'd created immediately prior to the last two MODEL/HIST units ...

After fiddling around for a while, I thought, "There are some blank entries in the HIST unit menu/list, why not add one and THEN add the MODEL and HIST?"

Which, indeed, worked ... the MODEL and HIST units now appeared on the drop down menu when I tried to connect the on-map UNIT to the latter.

So, if you encounter the same problem, evidently the HIST menu needs a blank spot every so often.